Small Craft Advisory 1000, NWS Anchorage, AK

Southcentral and Southwest Alaska Forecast Discussion
National Weather Service Anchorage AK
515 AM AKST Wed Dec 23 2020

.ANALYSIS AND UPPER LEVELS...

The upper-levels are dominated by a broad long-wave trough
extended from Siberia over almost the entirety of the Bering to
the western half of the state. Downstream, a ridge continues to
extend from the Canadian Rockies to the Brooks Range. This set up
is allowing shortwaves to eject from the long-wave trough and
move from the North Pacific into the Gulf of Alaska under the
influence of a deep southwesterly flow. The first of these waves
is moving onshore along the Southcentral coast early this
morning. At the surface, this feature is supporting a a 980mb low
over western Prince William Sound with a trough moving up Cook
Inlet into the southern half of the Susitna Valley. Colder air is
working in behind these features, allowing a wintry mix of
precipitation to change back to snow as the surface low lifts
north. Light snow is falling from Eagle River north across the
Mat-Su valleys this morning, but is already ending from Kenai
north to Anchorage. Elsewhere, the gusty winds along the immediate
coast are winding down as the surface front lifts onshore.

Out west, a second shortwave is moving toward the coast of
Southwest Alaska. This feature is producing snow showers from Bethel
to interior Bristol Bay as temperatures this morning range from
25F to 30F. Elsewhere, a vertically stacked 962 mb low just south
of Saint George is beginning to trek to the northeast. As it does,
scattered rain/snow showers continue from the Pribilofs down to
the central and eastern Aleutians. A secondary trough dropping
south along the back side of the low is producing gales, as seen
in the latest ASCAT pass, over the western Bering. Farther west, a
front attached to a 976 mb Kamchatka low is approaching the Attu
and Shemya.

&&

.MODEL DISCUSSION...

The biggest forecast challenge in the short term looks to be the
strength and position of the next North Pacific low as it lifts
northwest of Kodiak Island and potentially across the AKPEN by
Friday morning. The 00z GFS was the fastest and strongest,
bringing a more compact 980 mb low over Bristol Bay Friday
morning. The 00z NAM, GEM, EC all had a weaker solution that
either elongated the low at it interacts with the mountains along
the AKPEN, or kept the main circulation south of the AKPEN and
lifted an inverted trough over Bristol Bay. The 12z NAM is
sticking to its guns with the 06Z GFS moving more toward a NAM
solution, albeit still faster and stronger in its evolution. By
late Friday into Saturday, the model trends indicate this feature
will likely lift across the Southwest coast as another compact low
over the eastern Gulf of Alaska moves west to Prince William
Sound. Still, there are plenty of discrepancies in timing and
strength of these features between the models, limiting forecast
confidence by the weekend. For the Bering, Models are struggling
with the evolution of a complex low with multiple centers of
circulation as its moves from Kamchatka to the central Aleutians
by Friday. The NAM, GFS, and EC all show a similar trend with this
feature, but, again, there are timing and position issues with
the multiples lows, keeping forecast confidence average at best
by Friday.

&&

.AVIATION...
PANC...VFR ceilings and visibility will generally persist from
mid-morning today through early Thursday morning as a frontal
system lifts away from the airport complex early this morning.
Occasional MVFR / IFR ceilings are possible until 15z as a mix of
light rain and snow comes to an end. MVFR ceilings are then
expected to return by early morning Thursday as a second, weaker
trough moves up Cook Inlet toward Anchorage.

&&

.SHORT TERM FORECAST SOUTHCENTRAL ALASKA (Days 1 and 2: Today
through Thursday Night)...

Southcentral is rapidly approaching a much quieter period of
weather for most of the area. Note that quieter weather doesn't
necessarily mean quiet. Here is what we are looking at:

The last vestiges of the powerful storm of the past few days are
crossing the area this morning. The last of the snow has moved
through Anchorage, and the back edge of the snow will steadily
progress north across the Mat-Su over the next few hours. Mountain
snow showers across the Alaska Range, Talkeetnas (including
Hatcher Pass) and coastal Chugach Mountains will persist through
the day, with potential for isolated snow showers into the Copper
River Basin through the day.

Behind the low, we are seeing widespread low stratus and patchy
fog developing over portions of the western Kenai Peninsula this
morning. The fog will persist through the morning, and quite
probably longer. For other areas, the skies are clearing, and most
areas should see at least some broken sunshine this afternoon.
Temperatures are at or slightly above freezing in most areas of
the Cook Inlet Region. After 1-3 inches of snow overnight, the
above freezing temperatures will keep conditions very slick
through the day today, followed by cooling back into the 20s
tonight resulting in refreezing. Elsewhere, the onshore southerly
flow persisting into tonight will keep upslope snow ongoing as
well through Thompson Pass through tonight. However, the warm
southerly flow should keep the consistency of the snow wet and
heavy, thus despite the winds, any blowing snow should not result
in significant reductions in visibility.

On Thursday, the break in the weather will end on Kodiak Island. A
northwestward moving low will track well south and west of the
island during the day. This will lead to strong southeasterly
onshore flow developing across the island, leading to periods of
heavy rain through the day, with snow in the mountains. There is
some uncertainty on how far north the extent of that
precipitation will go, including as far north as the Homer area
and the southern Kenai Peninsula. This may result in rain shower
activity around Homer, but that is just a chance right now, and
it's definitely possible it stays dry there as well. The easterly
onshore flow will develop well north of the center, thus it is far
more likely that upslope precipitation (rain at the lower
elevations, snow in the mountains), will develop across the
eastern Kenai Peninsula. Precipitation there is not expected to be
particularly heavy, but may be steady at times. Elsewhere, dry
weather will continue as an odd east to west oriented upper level
ridge moves over Southcentral through Thursday.

&&

.SHORT TERM FORECAST SOUTHWEST ALASKA (Days 1 and 2...Wed through
Thu night)...

A long-lived storm from the central Bering is still trekking
toward Kuskokwim Delta to beyond YK-Delta by this evening through
overnight tonight. Shortwaves associated with this system
combined with cold air advection behind the storm will enhance the
potential for increased snowfall tonight, then tapering off by
Thu morning. Meanwhile, cloud remnants from this same storm and
the onset of another storm from the North Pacific, combined with
cold air advection from the eastern Bering, will most likely
favor snow over rain across Bristol Bay and AKPEN through the
forecast period. However, a slight chance of rain is feasible
should warm temperatures rise from the Gulf as the warm air
boundary pushes northward and west of the region through Thu
night. The mix snow to rain pattern will basically keep the
forecast period a bit cooler but relatively quiet in the next
couple days.

&&

.SHORT TERM FORECAST BERING SEA/ALEUTIANS (Days 1 and 2...Wed
through Thu night)...

The storm near Pribilof Islands will begin to lose its energy as
it drives closer to Kusko Bay this evening. The pattern for
snowfall continues across the Aleutians as cold air advection and
trailing cloud remnants from the storm passes through the
Aleutians today and tonight. The next complex storm from the
Northwest Pacific has already entered the southwestern Bering
early this morning. This storm will elevate winds, seas, and
snowfall through Thu morning before exiting the Aleutians Thu
night. An upper level ridge follows right behind this storm, with
the next frontal system approaching the western Bering Thu night.
Clearly, more weather is expected over the Aleutians especially
along the western and central Aleutians by Thu and Thu night.

&&

.MARINE (Days 3 through 5: Friday through Sunday)...
Gulf of Alaska: Models are in relatively good agreement that a
low will track into the eastern Gulf, keeping quiet conditions
over the western and central Gulf through Saturday morning. By
Saturday afternoon, models all agree that a North Pacific low will
track into the northern Gulf. However, there is still some
uncertainty in the strength of this system, which will likely
remain below gale force before moving inland by Sunday morning.
Behind this low, a strong front enters the western Gulf through
Sunday afternoon, which could bring a round of gale force winds to
the region.

Bering Sea: Models are struggling with the placement of a low over
the eastern Bering/AKPEN region on Friday afternoon, which could
see an area of gale force winds, before quickly weakening Friday
night. A much stronger, strengthening system farther west will be
the main feature to focus on starting Friday afternoon. The
associated gale force front will move over the central Aleutians
through Saturday morning. However, there is still model
uncertainty as to how strong this system will get as it moves
eastward through Saturday afternoon. Models agree that there will
be high end gale force winds, with the potential to even bring
storm force winds into the forecast area.

&&

.LONG TERM FORECAST (Days 3 through 7: Friday through Sunday)...
The long term forecast starting Friday looks to start with
continued persistence with a deep vertically stacked low over most
of the Bering, with weaker but quickly developing lows in the
Gulf. With deep southerly flow over most of the Mainland and
continued active weather, confidence is good that temperatures
should remain above normal with an increased risk for storms in
the Gulf through the upcoming weekend.

&&

.AFC WATCHES/WARNINGS/ADVISORIES...
PUBLIC...Flood Advisory 121 125
		 Winter Weather Advisory 101.
MARINE...Gale Warning 120 125 130-132 136-138 155 165 175-180 351
					  352 411 413 414.
FIRE WEATHER...NONE.

&&

SYNOPSIS AND MODEL DISCUSSION...TM
SOUTHCENTRAL ALASKA...JW
SOUTHWEST ALASKA/BERING SEA/ALEUTIANS...CB
MARINE/LONG TERM...AH